In exploration drilling, the average length of a drill string may typically be about 900 m. The drill string is typically composed of a plurality of drill rods, which, depending on configuration, typically weigh about 11-20 kg each and measure about 2-3 m in length. The drill rods are interconnected by a threaded connection.
Moreover, in many applications, also depending on rock type, tool type and drilling speed, it may be necessary to exchange the drill bit or other tool parts, e.g. every 300 m of drilling. Changing tools may be associated with retrieving the entire drill string from the hole, changing the lowermost portion and then reinserting the entire drill string, after which drilling may continue. In practice, and depending on rock conditions, 10-20 retrieval operations per drill hole is not uncommon.
Needless to say, a very large number of drill rods will need to be handled, including picking them from a transport carrier, inserting them into the drill, fastening them, releasing them and replacing them at the transport carrier.
In reality, this may mean that an operator has to carry/lift an 11 to 20 kg rod, about 1200 times to or from the rig for each hole. The estimated average number of holes drilled by one rig is 35 holes/year, resulting in that the operator carries (11 to 20)*1200*35/(220 working days)=2100 to 3820 kg/day. This is the main reason for developing a so-called Rod Handling System (RHS).
Such Rod Handling Systems are disclosed in WO2011/129760A1 and WO 00/65193A1. A Rod Handling System may typically comprise a robot arm having a dedicated gripper for gripping the drill rods. During a forward drilling operation, the robot arm is arranged to pick up drill rods at a transport or intermediate carrier and to place the drill rod in the drill unit, whereupon the drill rod is connected to an already installed drill rod to extend the drill string. During a drill string retrieval operation, the robot arm is arranged to pick up disconnected rods from the drill unit and to replace them onto the transport or intermediate carrier.
In order to provide a fully automatic system, thereby further eliminating manual work, it is desirable for the Rod Handling System to be able to connect and disconnect the drill rod to/from the installed drill rods.
However, the threads used in many drilling applications, including wire-line core drilling, may have a very low thread height, and they may be slightly conical.
If a pair of such threads is brought axially together at random, experience shows that there is about 60% chance of the threads not engaging each other, or engaging each other incorrectly. In either case, the threads may become damaged, resulting in additional cost and work.
WO 02/079603A1 discloses a system for automatically connecting drill rods to form a drill string. In this system, marks are provided around the perimeter of the rods, such that their rotational positions can be determined, thus allowing the rods to be rotationally aligned for optimal thread entry.
There is a need for an improved way of automatically and safely finding the thread entrance when using a Rod Handling System for connecting drill rods.